<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    *{
      padding:0;
      margin:0;
    }
    /* 浮动的元素会覆盖其他元素 */
    .box{
      width:100px;
      height:100px;
      background-color: green;
      float: left;
    }
    .son{
      width:200px;
      height:200px;
      background-color: red;
    }
  </style>
</head>
<body>
  <div class="box"></div>
  <div class="son"></div>
 
</body>
</html>